Biomedical subjects

Dimitri Chernyak

Publications and source records attributed to Dimitri Chernyak.

5 recordsLinked to original sources

PubMed2006

Predicting patients' night vision complaints with wavefront technology.

PURPOSE: To evaluate the accuracy of the diagnostic capabilities of optical metrics generated from wavefront measurements in relationship to post-laser-assisted in situ keratomileusis (LASIK) visual complaints as expressed and drawn by patients. DESIGN: Retrospective analysis and observational case series. METHODS: Patient wavefront data from an investigational device exemption study for wavefront-guided ablations were used to derive normative modulation transfer function (MTF), encircled energy (EE), and Strehl ratio. These optical metrics and their point-spread functions (PSF) were compared with data from five postoperative patients with night vision complaints. Patients were asked to draw their symptoms, which were elicited by testing with a Fenthoff muscle light, while using their best-corrected distance vision. RESULTS: The MTF, EE, and Strehl ratio of most patients were markedly different from those of the averages of 208 normal myopic eyes before and after LASIK surgery. The spatial extent of the PSF correlated positively with the severity of the visual complaints. Wavefront-derived PSFs were markedly similar to the patients' drawings. CONCLUSIONS: The results of this study demonstrate the diagnostic capability of the wavefront system in predicting visual symptoms and complaints of patients with high-order aberrations. Objective visual metrics from patients with night vision complaints were different from those of normal myopic eyes that had undergone LASIK procedures.

Adult↗

PubMed2006

Corneal asphericity and visual function after wavefront-guided LASIK.

PURPOSE: In recent years, a theory has been advanced that corneal asphericity in and of itself determines visual function; that the natural, optimal shape of the cornea is prolate; and that changing the cornea from prolate to oblate negatively impacts visual results. This article presents an analysis of the impact of corneal asphericity on wavefront-guided LASIK. METHOD: A retrospective analysis was conducted of 160 myopic eyes that had undergone wavefront-guided LASIK. Surgical procedures and data collection were conducted at six clinical sites. Corneal topography, visual acuity, and contrast sensitivity data were collected before and 6 months after surgery. The topographically measured corneal surface of each eye was fitted to a conic, and a Q-value was computed for a 5.5-mm pupil. Multivariate regression analysis was performed to evaluate the correlation between Q-value and visual function. The relationship of changes in the corneal surfaces to visual performance was also investigated. RESULTS: Preoperative corneas exhibited negative (prolate) conic shape constants. Postoperative corneas were about equally divided between positive (oblate) and negative conics. There was no statistically significant correlation between corneal shape and visual acuity or contrast sensitivity function. Changes in corneal asphericity after surgery had no significant correlation with changes in visual acuity or contrast sensitivity. CONCLUSION: Visual acuity and contrast sensitivity after wavefront-guided LASIK are not dependent on corneal asphericity. Neither preserving nor inducing asphericity ensures better visual outcome. Better visual outcomes are more likely to result from the application of a customized shape than a standard conic shape.

Adult↗

PubMed2005

On the discriminability of hROIs, human visually selected regions-of-interest.

Many studies have tried to answer an important question: is it possible to predict human visually selected regions-of-interest (hROIs)? hROIs are defined as the loci of eye fixations and they can be analyzed by their spatial distribution over the visual stimulus and their temporal ordering. We used a simplified set of geometrical spatial kernels and linear filter models as bottom-up conspicuity operators that produce algorithmically selected regions-of-interest, aROIs. As a direct approach we measured the ability of these aROIs to predict human scanpaths. The level of prediction is measured by two similarity indices: Sp for spatial similarity and Ss for temporal ordering similarity. At the same time we assessed the discriminability of the hROI loci, in terms of conspicuity, with respect to non-selected (not of interest) regions of an image. We prove that this discrimination is possible and further correlates with the positional similarity index Sp. Other human scanpath experimental conditions are presented in parsing diagrams and discussed. A general top-down/bottom-up scanpath model is finally formulated.

Algorithms↗

PubMed2005

Computational modeling of mechanical anisotropy in the cornea and sclera.

PURPOSE: To determine the biomechanical deformation of the cornea resulting from tissue cutting and removal by use of a new computational model and to investigate the effect of mechanical anisotrophy resulting from the fibrillar architecture. SETTING: Department of Mechanical Engineering, Stanford University, Stanford, California, USA. METHODS: A mathematical model for a typical lamella that explicitly accounts for the strain energy of the collagen fibrils, extrafibrillar matrix, and proteoglycan cross-linking was developed. A stromal model was then obtained by generalized averaging of the lamella properties through the stromal thickness, taking into account the preferred orientations of the collagen fibrils, which were obtained from x-ray scattering data. RESULTS: The model was used to predict astigmatism induced by a tunnel incision in the sclera, such as is used for cataract extraction and intraocular lens implantation. The amount of induced cylinder was in good agreement with published clinical data. Results show it is important for the model to incorporate preexisting corneal physiological stress caused by intraocular pressure. CONCLUSIONS: The mathematical model described appears to provide a framework for further development, capturing the essential features of mechanical anisotropy of the cornea. The tunnel incision simulation indicated the importance of the anisotropy in this case.

Anisotropy↗

PubMedDate not supplied

Corneal asphericity and retinal image quality: a case study and simulations.

PURPOSE: The optical quality of retinal images is dependent on the refracting elements of the eye including the nominally aspheric cornea and crystalline lens. This paper presents a retrospective theoretical analysis of the impact of corneal asphericity on the quality of retinal images. Clinical data are from the VISX Incorporated CustomVue IDE. METHOD: Topography, contrast sensitivity, and visual acuity data were collected from 278 myopic eyes before and after wavefront-guided laser surgery. The measured corneal surface of each eye was fitted to a conic, and a Q-value was computed for a 5.5-mm pupil. A model eye was used to simulate various amounts of optical asphericity. RESULTS: Preoperatively, most corneas exhibited negative conic shape constants. Postoperatively, corneas were about equally divided between positive and negative conics. There was no statistically significant correlation between the shape of the cornea and the subjects' perceptions of image quality including contrast sensitivity and visual acuity. Simulations showed that the corneal Q-value can vary from more to less prolate depending upon the shape of the internal surface. CONCLUSION: Following wavefront-guided laser in situ keratomileusis (LASIK), contrast sensitivity is usually good and is not dependent upon the corneal conic shape. Better visual outcomes are more likely with a customized shape than a standard best conic shape.

Adult↗